Climate Colder on Land During Last Ice Age than Thought

An episode 20,000 years ago in which the world cooled has been used as a guide for society to know how strongly climate can respond to variations in carbon dioxide levels. Researchers at Scripps Institution of Oceanography at UC San Diego and colleagues now report, however, that that account was far too conservative.

Tropical regions and the latitudes of Earth that would include most of today’s most populated regions cooled by roughly 6℃ (11℉ ) on land during an event scientists call the Last Glacial Maximum (LGM). The mainstream scientific community had previously concluded that the cooling had been below 2℃ (3.6℉) at the sea surface. There had been no clear consensus about cooling of the low-latitude land surface. 

Paleoclimatologists suspected for years that the planet actually reacted more strongly to the cooling event based on analyses of elements known as noble gases dissolved in groundwater. Such gases are ideal recorders of past temperature due to their completely unreactive nature. Within that niche of science, however, there were differing opinions about measurement methodology that prevented the community from presenting a truer picture of what happened.

Scripps paleoclimatologist Jeff Severinghaus said the disagreement was finally resolved but only now had researchers analyzed data using agreed-upon methods. The finding supports another recent analysis of sea-surface temperatures that produced largely similar results. 

The results appear in a study published May 12 in the journal Nature.

“The real significance of our paper is that prior work has badly underestimated the cooling in the last glacial period, which has lowballed estimates of the Earth’s climate sensitivity to greenhouse gases,” said Severinghaus, a study co-author. “The main reason that prior work was flawed was that it relied heavily on species abundances in the past. But just like humans, other species tend to migrate to where the climate suits them. Think, for instance, of snowbirds moving from Canada to Arizona in winter. So, species aren’t very good thermometers.”

The study is among several suggesting that what the world will experience as present-day climate continues to destabilize could be more drastic, and that many initial estimates from the climate science community, to the extent they have been inaccurate, have been so only because they were too conservative. 

“The rather high climate sensitivity that our results suggest is not good news regarding future global warming, which may be stronger than expected using previous best estimates,” said co-author Werner Aeschbach, a researcher at the Institute of Environmental Physics at Heidelberg University, Heidelberg, Germany. “In particular, our global review reinforces the finding of several single noble gas case studies that the tropics were substantially cooler during the last glacial maximum than at present. The unpleasant implication for the future is that the warmest regions of the world are not immune to further heating.”  

Much of the research originated in Severinghaus’ laboratory at Scripps Oceanography. Lead author Alan Seltzer, now a faculty member at Woods Hole Oceanographic Institution in Woods Hole, Mass., conducted field research for the study as a Scripps Oceanography student. In particular, Seltzer found evidence that the drastic cooling affected the San Diego area through his analysis of ancient groundwater samples collected from local monitoring wells, including clusters of wells next to Balboa Park and the former Qualcomm Stadium site.

The paper made use of a technique in which measurements of noble gases dissolved in ancient groundwater enable direct and quantitative determination of past surface temperature. Noble gases such as neon, argon, krypton, and xenon are chemically and biologically inactive in the atmosphere. They dissolve into groundwater, and their equilibrium concentrations depend strongly on temperature. The authors compiled four decades’ worth of groundwater noble gas data from every continent except Antarctica, along with previously unpublished measurements from some key tropical locations to produce a global record of noble gas-derived temperatures from the Last Glacial Maximum.

“Noble gas paleo temperature records are so powerful because they are based on a physical principle and are not much influenced by life—which always complicates everything—or by short-term extreme events,” said co-author Martin Stute, a researcher in the Environmental Science Department at Barnard College and an adjunct senior research scientist at the Lamont-Doherty Earth Observatory. “They provide a temperature average over hundreds to thousands of years. It is remarkable, and rewarding for me, how consistent noble gas paleo temperature reconstructions are in low latitudes from the early studies that I led in the 1990s to the most recent ones.”

The study bolsters the method of analyzing noble gases to reconstruct paleo temperatures and provides more confidence in climate models, according to the authors.

“Another key goal of our study was to evaluate the overall accuracy of the so-called ‘noble gas paleo-thermometer’ for reconstructing temperatures on land during the last glacial period,” said Seltzer. “Naturally, our ability to confidently use this tool to understand the past is related to how well it works in the present. By comparing modern temperature observations to independent estimates using noble gases in relatively young groundwater, we found that the noble gas thermometer is remarkably accurate over a wide temperature range from around 2 to 33⁰C (36 to 91⁰F). This adds a good deal of confidence to our estimates of cooling during the LGM.”

Seltzer added that the new analysis is important because climate models “provide an important tool that policy makers can use to decide on how to prepare for future environmental changes. This study alleviates the concern that, based on LGM proxy data, models might over-predict the global mean temperature response to carbon dioxide. In fact, based on both our study and the recent marine-proxy compilation, it is becoming clear that paleoclimate proxies and models are in agreement.”

Co-authors include Jessica Ng and Justin Kulongoski from Scripps and Rolf Kipfer of the Swiss Federal Institute of Aquatic Science and Technology.

Woods Hole Oceanographic Institution contributed to this release